1. Field of Invention
The present invention relates to a diamond sintered body which is applied to various tools such as a cutting tool, a drill bit and a wiredrawing die, and a method of manufacturing the same.
2. Description of the Prior Art
Now on the market is a sintered body for tools containing a diamond content in excess of 70 percent by volume in which diamond particles are bonded to each other. Such a sintered body is employed for cutting nonferrous metals, plastics or ceramics, or is applied to dressers, drill bits or wiredrawing dies. The diamond sintered body is remarkably excellent in efficiency particularly when the same is employed in cutting of nonferrous metals or is applied to wiredrawing dies of relatively soft wire rods such as copper wires.
Such diamond sintered bodies are provided in various structures and shapes depending on the application thereof, and the cutting tools, dressers and rock drilling bits are generally formed by those in which layers of diamond sintered bodies are bonded to highly tough base metals such as cemented carbide.
Known as examples of compound sintered compacts in the aforementioned structure are a compound sintered compact disclosed in U.S. Pat. No. 3745623 in which a layer of a diamond sintered body is directly bonded to a base material of WC-base cemented carbide, and that disclosed in U.S. Pat. No. 4403015 in which a layer of a diamond sintered body is bonded to a base material of cemented carbide etc. through an intermediate bonding layer.
In the diamond sintered body layer of the aforementioned compound sintered compact now in use, an iron group metal such as cobalt is generally employed as a binder for diamond particles. The iron group metal is utilized as a catalyst for synthesizing diamond from graphite, and is considered to partially dissolve the diamond powder in sintering under a superhigh pressure, thereby to securely sinter the layer of the diamond particles.
The iron group metal may be mixed with the diamond powder before sintering, and also known in the art is a method of infiltrating a solution of the base material of WC-Co in the diamond powder in sintering, as disclosed in U.S. Pat. No. 3745623. Such a diamond sintered body is excellent in abrasion resistance and strength, and highly efficient in the uses for which single-crystal diamond has generally been employed, whereas the same is greatly restricted with respect to heat resistance. Diamond is graphitized from its surface at a temperature of about 900.degree. C. and over in the atmosphere, while such graphitization is hardly caused in a vacuum or an inert gas even if the temperature is about 1400.degree. C. However, when the aforementioned general diamond sintered body is heated, efficiency of the tool is deteriorated at a temperature of about 750.degree. C. This means that the efficiency is naturally lowered under such a condition of use that the tool (e.g., a cutting tool or drilling bit) is used under a high temperature. The conventional diamond sintered body may be deteriorated at a temperature lower than that in the case of single-crystal diamond, for the reason that the diamond is largely different in thermal expansion coefficient from the iron group metal binder and the thermal stress in the sintered body is increased by heating whereby the texture is broken, and for the reason that the iron group metal facilitates graphitization of the diamond.
In order to improve heat resistance of the diamond sintered body, proposed is a method of preparing a sintered body which is not bonded to a base material such as cemented carbide and dipping the same in aqua regia or the like to be heated, thereby to leach the metallic bond phase in the sintered body, as disclosed in U.S. Pat. No. 4224380. It is considered that heat resistance of the diamond sintered body is improved to stand heating up to 1200.degree. C.
In the diamond sintered body disclosed in U.S. Pat. No. 4224380, about 5.about.30 percent by volume of pores are defined since the metallic bond phase is leached by acid treatment. Thus, the strength of the sintered body is so greatly lowered that the same is not sufficient in toughness for use as a tool. Further the method disclosed in the prior art, the sintered diamond body is not satisfactory for application to a drilling bit.
The inventors have already proposed a diamond sintered body which is excellent in strength, abrasion resistance and heat resistance as disclosed in Japanese Patent Laying-open Gazette No. 35066/1984. The sintered body disclosed therein is obtained by employing a carbide selected from groups IVa, Va and VIa of the periodic table as the binder to substantially decrease the content of pores, thereby to prevent lowering of strength of the sintered body caused by leaching of cobalt.
However, it has been recognized that although the diamond sintered body disclosed in Japanese Patent Laying-open Gazette No. 35066/1984 is reliably prevented from lowering of strength, the same is deteriorated by the difference in thermal expansion between the carbide and the diamond material under a high temperature over 1000.degree. C. Thus, the diamond sintered body is not yet satisfactory for application to a tool whose edge is exposed to a high temperature, such as that employed in geothermal well drilling.